Мне нравится то предложение, в котором нет лишних слов.

Because нравиться works differently from любить.

• Мне нравится X literally means X is pleasing to me
• Я люблю X means I love X or I like X very much

So in this sentence, Russian is using the common pattern:

• мне = to me
• нравится = is pleasing

This is a very normal way to say I like in Russian.

With нравиться, the person who feels the liking goes in the dative.

So:

• мне нравится = I like
• literally: to me it is pleasing

That is why Russian does not use я here. The sentence is built around the idea that the sentence itself is what pleases the speaker.

Because the grammatical subject is то предложение, which is singular.

Russian makes нравиться agree with the thing that is pleasing, not with the person.

• Мне нравится предложение = I like the sentence
• Мне нравятся предложения = I like the sentences

Since предложение is one sentence, you get нравится.

То means that and points to a specific sentence, especially one that is then described by the relative clause.

So:

• то предложение = that sentence
• то предложение, в котором нет лишних слов = that sentence in which there are no unnecessary words

In English, this often sounds natural as the sentence that has no unnecessary words.

Russian often uses тот, та, то, те to set up a noun that will be further defined.

Yes, it often can.

You could say:

• Мне нравится предложение, в котором нет лишних слов.

This is also natural and probably a bit more neutral.

Adding то gives a slight sense of pointing out a particular one, something like:

• I like that sentence, the one that has no unnecessary words

So both are possible, but то adds a bit of emphasis or specificity.

Который is the relative pronoun meaning which or that.

Here it refers back to предложение. It must agree with предложение in:

• gender: neuter
• number: singular

That is why you get the neuter singular form котором.

Its case is decided by its role inside the relative clause. Since the phrase is в котором = in which, the preposition в requires the prepositional case here.

So:

• предложение → neuter singular
• в котором → in which

You sometimes can hear где in everyday speech:

• предложение, где нет лишних слов

But в котором is more standard and more precise here.

Why? Because the idea is literally in the sentence, so Russian uses the more exact relative construction:

• в котором = in which

Где literally means where, and although Russian often extends it beyond physical places, many teachers and careful writers prefer в котором in this kind of sentence.

Russian usually does not use a present-tense form of to be the way English does.

So instead of saying something like in which there are no extra words, Russian simply says:

• в котором нет лишних слов
• literally: in which no extra words are not-present

The key word is нет, which means there is not / there are not / there is no / there are no depending on context.

Because нет requires the genitive case.

The dictionary forms are:

• лишние слова = unnecessary words

But after нет, both words change to genitive plural:

• лишних слов

So:

• слова → слов
• лишние → лишних

This is a very important Russian pattern:

• есть лишние слова = there are unnecessary words
• нет лишних слов = there are no unnecessary words

It comes from the adjective лишний, which can mean:

• extra
• unnecessary
• superfluous

In this sentence, лишних слов means unnecessary words or extra words.

Because it follows нет, it appears in the genitive plural form лишних.

It breaks down like this:

• Мне нравится = I like
• то предложение = that sentence
• в котором нет лишних слов = in which there are no unnecessary words

So the structure is:

• main clause: Мне нравится то предложение
• relative clause: в котором нет лишних слов

The relative clause describes предложение.

Yes, Russian word order is flexible, though some orders sound more neutral than others.

The original sentence is natural and clear:

• Мне нравится то предложение, в котором нет лишних слов.

You could also say:

• Мне нравится предложение, в котором нет лишних слов.

Other rearrangements are possible for emphasis, but the original is a good standard version for learners. The main thing is to keep в котором together and keep the relative clause attached clearly to предложение.